Method and apparatus for improving the efficiency and accuracy of rfid systems

ABSTRACT

The present invention relates to a method and apparatus for transmitting a narrow signal beam that allows the precise location of RFID tags to be determined and reduces tag collisions. The present invention further relates to a method and apparatus for combing an RFID reader with an optical source to visualize the interrogation zone of the reader. The present invention also relates to a method and apparatus for improving the efficiency of RFID systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/550,355, filed Mar. 5, 2004, U.S. Provisional Application No.60/550,411, filed Mar. 5, 2004, U.S. Provisional Application No.60/561,433, filed Apr. 12, 2004, U.S. Provisional Application No.60/603,531, filed Aug. 20, 2004, U.S. Provisional Application No.60/613,428, filed Sep. 27, 2004, and U.S. application Ser. No.11/066,048, filed Feb. 25, 2005, each of which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

Radio Frequency Identification (“RFID”) is a generic term fortechnologies that use radio waves to automatically identify individualitems. Objects can be identified using RFID by storing a serial numberthat identifies the object on a chip that is attached to an antenna. Thechip and the antenna together are called an RFID tag. An RFID readersends out electromagnetic waves that are received by the antenna on theRFID tag. Passive RFID tags draw power from this electromagnetic fieldto power the chip. Active tags use their own batteries to power thechip. The tag responds to the reader by transmitting a bit stream to thereader that contains information about the tag (serial number, etc.).The current state of RFID technology is described by [1] K. Finkenzellerin “RFID Handbook” (John Wiley & Sons, 2003).

RFID systems operate at many different frequencies. The most common arelow frequencies around 135 KHz, high frequencies around 13.56 MHz,ultra-high frequencies around 900 MHz, and microwave frequencies around2.45 GHz and 5.8 GHz.

Current RFID systems are not suited for precise location of tags becausereaders transmit very broad beams that cause tags in a large region torespond. Moreover, when two or more tags respond simultaneously, thetransmissions from the tags get scrambled and become unintelligible tothe reader. This phenomenon is known as tag collision. Procedures thatinvolve repeated transmissions of tag data have been developed to dealwith tag collisions [1, Chapter 7]. However, the interrogation speed(number of tags interrogated per second) is reduced when a large numberof repeated transmissions are needed, so it is desirable to reduce tagcollisions as much as possible.

The RFID reader's efficiency is related to its coverage or “accuracy,”which is measured by the percentage of tags within range that are readcorrectly. The accuracy of today's readers is not acceptable for manyapplications, which require 100 percent accuracy. For example, a studypublished in the article “Smart Tags for Your Supply Chain,” McKinseyQuarterly, 2003, Number 4, found that RFID-tagged pallets failed 3percent of the time even when double-tagged, and only 78 percent of theindividually tagged pallets were read accurately.

According to the article “RFID will present a stiff test,” published inSupply Chain Management Review, Jan. 15, 2004, the main cause of lowreader accuracy is the inability of readers to transmit enough power toactivate tags that are surrounded by other objects such as tags affixedto items stored in the middle of a pallet. The article reports that adhoc repositioning of the RFID tags or increasing reader power can oftenfix this problem.

The problem of reader collisions is another barrier to the large-scaledeployment of RFID. Reader collisions can occur when the interrogationzones of two or more readers overlap. In the article “Why UHF RFIDSystems Won't Scale,” RFID Journal, July 2004, H. L. van Eeden statesthat “The main technical problem facing end-user companies is thepossibility of large-scale reader interference that could render UHFRFID installations completely inoperable and severely limit the rolloutof UHF RFID systems.”

The problems of reader collision and low reader accuracy are related: ifone attempts to solve the problem of low reader accuracy by increasingthe reader power, then the interrogation zones grow and readercollisions become more frequent.

The following five U.S. Provisional Applications describe RFID readersthat transmit data signals that cause the tags to respond and scramblesignals that do not cause the tags to respond: [2] “Method and apparatusfor secure transmission of data using array,” U.S. ProvisionalApplication No. 60/550,355, filed Mar. 5, 2004, [3] “Method andapparatus for preventing unauthorized transmitters from gaining accessto a wireless network,” U.S. Provisional Application No. 60/550,411,filed Mar. 5, 2004, [4] “Method and apparatus for precise location ofRFID tags,” U.S. Provisional Application No. 60/561,433, filed Apr. 12,2004, [5] “Optically guided reader of RFID tags,” U.S. ProvisionalApplication No. 60/603,531, filed Aug. 20, 2004, and [6] “Method andapparatus for improving the efficiency of RFID systems,” U.S.Provisional Applications No. 60/613,428, filed Sep. 27, 2004. These fiveprovisional applications are incorporated herein by reference in theirentirety.

The data and scramble signals are transmitted with different beams thatare adjusted such that the scramble signals overshadow the data signalsin all but selected regions. Hence, a tag will respond only if it islocated in one of the selected regions, called the interrogation zones.

Provisional patent application [2] describes methods for using sum anddifference patterns of array antennas to transmit data into selectednarrow angular regions. The data signal is shielded by a scramble signalthat makes the total transmitted signal unintelligible everywhere exceptin the narrow angular region. The scramble signal is also allowed tocontain its own data that is different from the data carried by the datasignal. Provisional patent application [2] further describes how theprecise angular positions of RFID tags can be determined. Provisionalpatent application [4] describes how the width of the interrogation zonecan be reduced and how the absolute location of a tag can be obtainedfrom triangulation. Provisional patent application [5] describes how theinterrogation zone can be visualized with optical sources. Provisionalpatent application [6] describes how the efficiency of RFID readers andreader networks can be improved through measurements, modeling, andinversion.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a method for interrogating RFIDtags comprising transmitting a data beam that causes the tags torespond, transmitting one or more scramble beams that do not cause thetags to respond, and adjusting the data beam and the scramble beams suchthat the scramble beams overshadow the data beam in all but selectedregions. A tag can respond to the data signal either by broadcasting orchanging its stored information. A scramble beam can contain a separateintelligible data stream and can charge tags that are not beinginterrogated.

In one embodiment of the present invention, the data signal bits aredivided into two or more parts. For each part of the data signal, acorresponding scramble-beam direction is defined that is slightly awayfrom the direction of the data beam. Each part of the data signal isthen transmitted while the scramble beam has its central null steeredtowards a corresponding scramble-beam direction. The division of thedata signal must be such that a tag responds only if it receives all thedata bits.

In one embodiment of the present invention, the scramble signal is asine wave. In one embodiment of the present invention, the scramblebeams have approximately constant amplitudes away from their centralnull, so that the total radiated power from the reader is approximatelyomni-directional. Constant-amplitude scramble beams are achieved bymoving zeros far off the Schelkunoff unit circle or by iterativemethods.

In one embodiment of the present invention, the method further comprisesemploying two or more array readers that scan an area with data andscramble beams to determine the angular positions of each tag withinrange. In one embodiment the angular positions obtained with two or morereaders determine the absolute position of the tags throughtriangulation. In one embodiment, anti-collision methods are employedwhen more than one tag responds at any given scan angle.

In one embodiment of the present invention, the reader and tags areinductively coupled and the reader employs two or more loops to transmitdata and scramble signals. In one embodiment of the present invention,the loop configuration of the reader is optimized with iterativetechniques to ensure that the magnetic field of the data signal isovershadowed by the magnetic field of the scramble signal except inselected regions.

In one embodiment the security measures described in [2] and [3] areemployed to enhance the security of the RFID system.

The present invention is further directed to a method for opticallydisplaying the interrogation zone of an RFID reader that includes thesteps of attaching an optical source to an RFID reader and transmittingone or more light beams with said optical source to visualize theinterrogation zone. In one embodiment of the present invention, thelight beam is pointed in the direction of the center of theinterrogation zone. In another embodiment of the present invention, twoor more light beams are transmitted such that each light beam coincideswith a boundary of the interrogation zone.

In a further embodiment of the present invention, a light beam isscanned back and forth between the boundaries of the interrogation zone.In another embodiment of the present invention, the light beams aretransmitted with lasers.

In one embodiment of the present invention, the optical source is builtinto the housing of the RFID reader. In another embodiment of thepresent invention, the optical source is attached to the housing of theRFID reader.

The present invention is additionally directed to methods for improvingthe efficiency of RFID systems. In one embodiment of the presentinvention, the reader employs two antennas that broadcast both data andscramble signals. In one embodiment of the present invention, theantennas are patch antennas. In one embodiment of the present invention,the reader employs three antenna elements where two of them transmit thescramble signal and one interrogates the tags. In one embodiment of thepresent invention, the reader employs one standard commerciallyavailable reader and two additional antennas that broadcast a scramblesignal. In one embodiment of the present invention, the array excitationcoefficients for the data and scramble signals are adjusted to create aninterrogation beam that precisely fits an opening in a container.

In one embodiment of the present invention, the reader employs anantenna that transmits two or more interrogation beams designed suchthat any tag in the interrogation zone receives sufficient power tooperate from at least one of the interrogation beams. In one embodimentof the present invention, the reader employs two or more scramble beamsto prevent leakage of the data signal. In one embodiment of the presentinvention, two sets of scramble-beam coefficients are mirror images.

In one embodiment of the present invention, a network of readerstransmits both data and scramble beams adjusted to create closely spacedindependent interrogation zones. In one embodiment of the presentinvention, the positions of the readers is determined from the solutionof an inverse source problem. In one embodiment of the presentinvention, the excitation coefficients are determined from the solutionto an inverse source problem. In one embodiment of the presentinvention, the inverse source problem is solved with an iterativeoptimization scheme.

In one embodiment of the present invention, the tags are placed atlocations where the tag antenna creates maximum disruption of the fielddistribution. In one embodiment of the present invention, the fielddistribution on an object is computed with a numerical method.

In one embodiment of the present invention, an RFID reader uses abistatic mode of operation. In one embodiment of the present invention,the RFID reader uses a multistatic mode of operation. In one embodimentof the present invention, the location of the RFID reader receivers aredetermined by solving a scattering problem with a model for a typicaltagged item.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an 18-element linear array.

FIG. 2 illustrates a sum pattern of the 18-element array evaluated atθ=90°. The excitation coefficients are shown above the plot.

FIG. 3 illustrates a difference pattern of the 18-element arrayevaluated at θ=900. The excitation coefficients are shown above theplot.

FIG. 4 illustrates the sum and difference patterns of the 18-elementarray evaluated at θ=900. Both sets of excitation coefficients are shownabove the plot.

FIG. 5 illustrates a square planar array with 324 elements.

FIG. 6 illustrates a mapping of the excitation coefficients for the sumpattern of the 324-element planar array.

FIG. 7 illustrates a 3-D mapping of the array sum pattern correspondingto the excitation coefficients.

FIG. 8 illustrates a mapping of the excitation coefficients for thecosine difference pattern of the 324-element planar array.

FIG. 9 illustrates a 3-D mapping of the array difference patterncorresponding to the excitation coefficients.

FIG. 10 illustrates a circular ring a ray.

FIG. 11 is a perspective view of a reflector antenna with its feed atthe focal point.

FIG. 12 illustrates a linear array with 4 elements having elementspacing equal to 10 cm.

FIG. 13 illustrates data and scramble beams of the array in FIG. 12evaluated at θ=90°. The array elements are z-directed dipoles and bothsets of excitation coefficients are shown above the plot.

FIG. 14 illustrates data and scramble beams of the array in FIG. 12evaluated at θ=90°. The array elements are patch antennas and both setsof excitation coefficients are shown above the plot.

FIG. 15 illustrates one data beam and two scramble beams of the array inFIG. 12 evaluated at θ=90°. The array elements are patch antennas andboth sets of excitation coefficients are shown above the plot.

FIG. 16 illustrates a linear array with 8 elements having elementspacing equal to 6.25 cm.

FIG. 17 illustrates one set of data and scramble beams of the array inFIG. 16 evaluated at θ=90°. The array elements are z-directed dipolesand both sets of excitation coefficients are shown above the plot.

FIG. 18 illustrates another set of data and scramble beams of the arrayin FIG. 16 evaluated at θ=90°. The array elements are z-directed dipolesand both sets of excitation coefficients are shown above the plot.

FIG. 19 illustrates a scanning-array positioning system that employs twoarray-antenna readers.

FIG. 20 illustrates an inductive RFID reader employing two loopsparallel to the x-y plane.

FIG. 21 illustrates the magnitudes of the z-components of the magneticfields for the data and scramble signals transmitted by the inductivereader in FIG. 20.

FIG. 22 illustrates an RFID tag reader with an optical source attachedto its housing.

FIG. 23 illustrates the ratio of the scramble signal and data signalsfor a four-element array reader operating around 900 MHz. The datasignal dominates in the black shaded zone. The two light beams mark theboundaries of the interrogation zone.

FIG. 24 illustrates the geometry for an array of two antenna elementsthat can be tilted independently with respect to the array axis.

FIG. 25 illustrates a schematic of RF control electronics for atwo-element array. Each antenna element is driven by a linearcombination of two RF signals: a data signal and a scramble signal. Thebeam patterns for each signal are determined by the weightingcoefficients (in boxes); phase shifts (time delays) can be used to steerthe total beam pattern in a specific direction.

FIG. 26 illustrates free-space signal strength of data beam (Left) andscramble beam (Right). Axis units are meters.

FIG. 27 illustrates interrogation zones of data beam (Left) and ofcombined data and scramble beams (Right).

FIG. 28 illustrates total signal strength of the data beam (Left) andthe scramble beam (Right) when the beams are broadcast toward a concretewall.

FIG. 29 illustrates interrogation zones of data beam (Left) and ofcombined data and scramble beams (Right) when the reader broadcaststoward a concrete wall.

FIG. 30 illustrates geometry for an array of three identical patchantennas. The middle antenna broadcasts the data signal. The outerantennas broadcast scramble signals. The two outer elements are tiltedby the angle α.

FIG. 31 illustrates a schematic of RF control electronics for athree-element array. The middle antenna element is driven by the datasignal. The two outer elements are driven by the scramble signal. Thebeam patterns for each signal are determined by the weightingcoefficients (in boxes).

FIG. 32 illustrates free-space signal strength of data beam (Left) andscramble beam (Right). Axis units are meters. The distance between arrayelements is d=17 cm, and the tilt angle is α=30°.

FIG. 33 illustrates interrogation zones of data beam (Left) and ofcombined data and scramble beams (Right).

FIG. 34 illustrates the geometry for an array of four identical antennaelements.

FIG. 35 illustrates a schematic of RF control electronics for afour-element array. Each antenna element is driven by a linearcombination of two RF signals: a data signal and a scramble signal. Thebeam patterns for each signal are determined by the weightingcoefficients (in boxes); phase shifts (time delays) can be used to steerthe total beam pattern in a specific direction.

FIG. 36 illustrates the geometry of a 3D model of an RF tag reader(array), operating in front of a conveyor belt in a room with a metalwall and concrete floor and ceiling.

FIG. 37 illustrates interrogation zones above the conveyor belt wherethe data signal is more than 10 dB greater than the scramble. The topplot shows the interrogation zone for a simple superposition of one databeam and one scramble beam. Because of multi-path effects, the scramblebeam has nulls that cause leakage of the data signal into undesiredregions. The bottom plot shows the results of a more sophisticatedscheme that uses two scramble beams to eliminate leakage.

FIG. 38 illustrates top and side views of a 3D model of a networkconsisting of two RF tag readers and one scramble transmitter that eachuses a 2-element antenna array. The readers operate in front of aconcrete wall in a room with concrete floor and ceiling.

FIG. 39 illustrates signal strengths of the data beams of the tworeaders in FIG. 38. Reflections in the wall, floor, and ceiling areincluded. Axis units are in meters.

FIG. 40 illustrates interrogation zones of the data beams of the tworeaders in FIG. 38 when they broadcast toward the concrete wall. Readercollision occurs near the origin. Both readers set off tags on bothconveyer belts so it is not possible to determine which conveyer beltcarried a given item.

FIG. 41 (Left) illustrates the ratio of total scramble beam (obtainedwith the scramble transmitter and the two readers) to data beam of thereader on the left. FIG. 41 (Right) illustrates the ratio of totalscramble beam to data beam of the reader on the right. Tags areinterrogated only in the regions where a data beam dominates.

FIG. 42 illustrates two independent interrogation zones obtained withthe combined data and scramble beams of the scramble transmitter and thetwo readers. Reader collision is avoided, and it is possible to tellwhich conveyer belt carries a given item.

FIG. 43 illustrates a 2D model of a reader that interrogates a tagplaced on a high-dielectric object. The field of a line source (thereader) illuminates a dielectric cylinder (bottle containing a liquid).A conducting wire (the tag) is close to the surface of the cylinder.

FIG. 44 illustrates the total field in the vicinity of the dielectriccylinder when the wire is removed. The white circle marks the surface ofthe cylinder.

FIG. 45 illustrates the total field in the vicinity of the dielectriccylinder when the wire is placed on the side of the cylinder (as seenfrom the reader).

FIG. 46 illustrates the total field in the vicinity of the dielectriccylinder when the wire is placed on the back of the cylinder (as seenfrom the reader).

FIG. 47 illustrates the difference far field for tag placed on the sideof the cylinder. The amplitude of the backscattered field is very low.

FIG. 48 illustrates the difference far field for tag placed on the backof the cylinder. The amplitude of the backscattered field is large.

FIG. 49 illustrates a Bistatic RFID reader with a transmitter and areceiver that interrogates a collection of tags that are placed on itemsin a box.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides (a) designs for RFID readers, (b) amethod for reducing the width of the interrogation region, (c) a methodfor providing a user defined interrogation zone for one or more readers,(d) a method for location of transceivers in 2D and 3D using more thanone information-steering transmitter, (e) a method for precise taglocation that works in the induction regime where the wavelength is muchlonger than the physical dimensions involved, (f) a new set of securitymeasures for RFID systems, (g) a method for optically displaying theinterrogation zone, (h) a method for overcoming multipath effects, (i) amethod for optimal tag placement, and (j) a bistatic RFID reader. Acommon feature in items (a)-(f) is that two or more signals aretransmitted simultaneously, including:

-   -   1. A data signal that causes the tags to respond. The data        signal may instruct the tags to broadcast or modify stored        information. The data signal may contain information about scan        angles that the tags can retransmit back to the reader. Also,        the data signal may employ any of the methods developed to solve        the problem of tag collision that occurs when two or more tags        transmit simultaneously [1, Chapter 7].    -   2. One or more scramble signals that do not cause the tags to        respond. The tags neither broadcast nor modify their stored        information. A pure sine wave works as a scramble signal for UHF        tags. The scramble signals can be used to charge the tags and to        convey a separate intelligible information stream. The scramble        signal can also be referred to as a guard signal.

A reader is said to employ information steering when it transmits bothdata and scramble signals. The present invention makes extensive use ofantenna arrays. The following references describe the theory and designof phased arrays: R. C. Hansen, “Phased Array Antennas,” John Wiley &Sons, 1998; R. J. Mailloux, “Phased Array Antenna Handbook,” ArtechHouse, 1994; and, R. S. Elliot, “Antenna Theory and Design,” IEEE Press,2003. With adaptive phased arrays, also known as smart antennas, thereceived signals and environmental parameters are fed to powerfulprocessors that steer the beams to optimize performance. The technologyfor designing and constructing adaptive phased arrays with hundreds ofelements that produce prescribed sum and difference patterns has reacheda mature stage, as described in the following references: M. I. Skolnik,“Radar Handbook,” McGraw-Hill, 1990, 2nd edition; R. T. Compton,“Adaptive Antennas,” Prentice-Hall, 1998; and, G. V. Tsoulos, ed.“Adaptive Antennas for Wireless Communications,” IEEE Press, 2001.

Two types of array patterns widely used in radar applications are ofparticular interest to the present invention: (1) the sum pattern and(2) the difference pattern, the relevance of which will be seen in thecontext of FIG. 1.

Linear Arrays

FIG. 1 is a graph of a linear array with 18 elements having elementspacing of half of a wavelength. The data time signal is represented bya(t), which depends on the chosen modulation and coding techniques, andon the transfer functions of the antenna elements. The present inventionworks for any modulation and coding techniques and for any set of arrayelements.

In standard operation, one would feed array element #p with a signal ofthe form:

T _(p)(t)=A _(p) a(t−α _(p))

where A_(p) is the excitation coefficient and α_(p) is the time delay.Generally, one could feed each array element with time functions thathave different time dependence to compensate for array imperfections,end-element effects, or array elements differences. Such adjustmentswould be well known and straightforward to those working in this area.Therefore, it is assumed presently that the time dependence of eachinput signal is the same (the amplitudes and time delays are different).The array excitation coefficients and time delays (A_(p) and α_(p), p=1,2, . . . , N, where N is the number of elements) are determined bystandard methods to achieve a desired radiation pattern of the arraythat adapts to its environment.

FIG. 2 is a graph of a sum pattern of the 18 element array in FIG. 1,evaluated at θ=90°. A typical array radiation pattern and the associatedexcitation coefficients are shown in FIG. 2 for the 18-element lineararray shown in FIG. 1. The element spacing is equal to half awavelength. All time delays are zero, so the array operates in broadsidemode. Since all the excitation coefficients have the same sign, thearray radiates a sum pattern, which is characterized by a main beam andside lobes that are below a certain level (−55 dB in the embodimentshown in FIG. 2). Beam steering can be achieved by assigning nonzerovalues to the time delays, which results in the complex excitationcoefficients A_(p)exp(i2πf₀α_(p)) when the exp(−i2πf₀t) time dependenceis suppressed and f₀ is the frequency at which the sum pattern isevaluated. These issues are well known to those working in this area.

In addition to the sum pattern, a difference pattern may be broadcast. Adifference pattern and the associated excitation coefficients are shownin FIG. 3 for the 18-element array shown in FIG. 1. The differencepattern has a deep null in the center that is surrounded by two steeppeaks. The term “difference pattern” is used because half of theexcitation coefficients are positive and the other half negative. Thetime delays in FIG. 3 are all zero. As seen for the sum pattern, beamsteering can be achieved by assigning nonzero values to the time delays.

A narrow interrogation zone is obtained with the present invention byfeeding each element with a total signal that is obtained by adding atleast one scramble signal to the data signal. In the case of onescramble signal b(t), the total input signal to array element #p is:

T _(p)(t)=A _(p) a(t−α _(p))+B _(p) b(t−β _(p))

where B_(p) (p=1, 2, . . . , N) are the excitation coefficients andβ_(p) (p=1, 2, . . . , N) are time delays for the scramble signal. Thisarrangement of signals creates a narrow interrogation zone when the dataexcitation coefficients A_(p) (p=1, 2, . . . , N) produce a sum patternand the scramble excitation coefficients B_(p) (p=1, 2, . . . , N)produce a difference pattern. To steer the sum and difference beams inthe same direction, one simply sets α_(p)=β_(p).

To see how the narrow interrogation zone is provided, the sum anddifference patterns of FIGS. 2 and 3 are plotted together as shown inFIG. 4. The data signal a(t) is transmitted through the sum pattern, andthe scramble signal is transmitted through the difference pattern. Anobserver located at the nulls of the difference pattern will receiveonly the data signal. Conversely, an observer located at nulls of thesum pattern will receive only the scramble signal. In most locations,however, there are no nulls and an observer would receive a weighted sumof the data and scramble signals. The weights are simply the sum anddifference patterns at that particular location.

As shown by way of example in FIG. 4, it is evident that everywhereoutside the narrow angular region 87°<φ<93°, the difference pattern isgreater in magnitude than the sum pattern. Hence, an observer located atthe angle φ will receive the following signals:

-   -   φ=90°: the pure data signal a(t).    -   87°<φ<93°: a weighted sum of data and scramble signals in which        the weight for the data signal is greatest.    -   0°<φ<87° or 93°<φ<180°: a weighted sum of data and scramble        signals in which the weight for the scramble signal is greatest.

Consequently, in this example, only observers in the narrow angularregion 87°<φ<93° will understand the data signal. Additionally, theangular region in which the data signal can be understood is likely evennarrower due to noise.

Planar Arrays

The present invention may also be used with planar arrays such as the324-element array (18 elements by 18 elements) shown in FIG. 5. Theelement spacing is half a wavelength. FIG. 6 shows a typical set of sumexcitation coefficients, and FIG. 7 shows the corresponding array sumpattern. The array pattern is almost independent of φ and has a mainbeam in the broadside direction. Standard methods, as previously noted,can be used to steer the beam in any desired direction.

For planar arrays, the difference patterns with sharp nulls have cos(φ)or sin(φ) angular dependence. The φ independent difference patterns forplanar arrays result in a broadening of the angular regions in which thesignals are intelligible. FIG. 8 shows a set of difference excitationcoefficients with cos(φ) angular dependence, and FIG. 9 shows thecorresponding difference pattern.

The excitation coefficients for both the sum and difference patterns forthe planar array may be obtained with semi-analytical methods to achieveprescribed side lobe levels and main beam widths. Alternatively, thecoefficients may be obtained with nonlinear optimization techniques. Thecoefficients as shown in FIGS. 6 and 8 are obtained with the MATLAB™function FMINUNC, which minimizes a user-defined cost function. The costfunction is designed to ensure that the side lobes are below a certainlevel for all φ.

The difference pattern shown in FIG. 9 has a null for φ=90° and φ270°.With only one difference beam, the data signal leaks out at observationpoints with φ=90° or φ=270°. Therefore, at least two difference beamsare used for a planar array. The excitation coefficients and arraypattern for a sin(φ) difference pattern are obtained by rotating theplots as shown in FIGS. 8 and 9 ninety degrees around the z axis. Theinput to array element #p is therefore a sum of the three terms:

T _(p)(t)=A _(p) a(t−α _(p))+B _(p) b(t−β _(p))+C _(p) c(t−χ _(p))

where B_(p) and C_(p) are the excitation coefficients, β_(p) and χ_(p)are time delays, and b(t) and c(t) are the scramble signals applied tothe cos(φ) and sin(φ) difference patterns, respectively (p=1, 2, . . . ,N). As before, A_(p) (p=1, 2, . . . , N) are the excitation coefficientsand α_(p) (p=1, 2, . . . , N) are the time delays for the data signal.With at least two independent scramble signals, one achieves a narrowinterrogation zone around θ=0°. To steer the sum and difference beams inthe same direction, one simply sets α_(p)=β_(p)=χ_(p).

Other Antennas

User defined interrogation zones in accordance with the presentinvention may also be achieved with arrays that are neither linear norplanar. For example, the circular ring array shown in FIG. 10 is usefulfor providing 360° coverage. For ring arrays, the interrogation zonescan be obtained with sum and difference patterns obtained from standardtheory. Similar interrogation zones can be realized with reflectorantennas as shown in FIG. 11 by applying the present invention to itsfeed, which is typically a smaller antenna located at the focal point.More generally, one may use the present invention for any antenna typeto obtain sum and difference patterns that can be combined to achievethe desired interrogation zones.

For purposes of illustration, the examples herein are confined to sumand difference patterns because such patterns have been studiedextensively in the radar literature. Interrogation zones in accordancewith the present invention can be achieved, however, with anycombination of array patterns in which one of the patterns (the“difference pattern”) has a null in the direction of the tags ofinterest and is larger in magnitude than the other pattern (the “sumpattern”) in directions where other tags may be present.

The difference patterns must be slightly broader than the sum patternsto achieve the desired interrogation zones. The numerical examplespresented herein demonstrate that difference patterns may be designed tohave beam widths that are just slightly broader than the beam widths ofthe corresponding sum patterns.

A Four-Element Reader

Consider a four-element tag reader operating at frequencies around 900MHz. (RFID systems are allowed to operate at 915 MHz in the UnitedStates and at 869 MHz in Europe.) FIG. 12 shows a four-element array fora hand-held tag reader operating around 900 MHz, with element spacing=10cm and total array length=30 cm. The tags to be interrogated are nearthe (θ,φ)=(90°, 90°) direction. As in [2], one feeds element #p with asignal of the form:

T _(p)(t)=A _(p) a(t)+B _(p) b(t)

where a(t) is the data signal and b(t) is a scramble signal. A_(p) andB_(p) are the excitation coefficients for the data and scramble signals,respectively.

FIG. 13 shows the far field of the reader for the case where the arrayelements are z-directed dipoles with array excitation coefficients givenabove the plot. This design results in a data-signal width of about 30degrees. Hence, only those tags that are located in a 30 degree regioncentered on φ=90° will respond to this reader, allowing the operator tonarrow the region in which a given tag is located. Since the z-directeddipoles are omni-directional in the θ=90° plane, the array radiatesequal amounts of power in the forward and backward directions.

The present invention also contemplates a design for a hand-held readerthat radiates little energy in the backward direction (towards theoperator). Assume that the array elements are made of patch antennaswith sin²(φ) radiation patterns in the forward direction and very lowradiation pattern in the backward direction. FIG. 14 shows the field ofthis reader for forward directions 0<φ<180°. The patterns of the patchantennas ensure that the field in the backward directions 180°<φ<360° islow.

A Method for Reducing the Width of the Data Beam

The width of the angular region of the data signal is reduced bydividing the data signal bits into two parts: the first part and thesecond part. The first part is transmitted while the scramble beam hasits central null steered slightly to one side of the direction of thedata beam. The second part of the data signal is transmitted while thescramble beam has its central null steered slightly to the other side ofthe direction of the data beam. The division of the data signal must besuch that a tag responds only if it receives both the first and secondpart of the data signal.

FIG. 15 shows how this method can be implemented with the four-elementarray in FIG. 12, with patch antennas. Scramble beam #1 is obtained bysteering the central null to φ=99°. Scramble beam #2 is obtained bysteering the central null to φ=81°. The excitation coefficients areprovided above the plot. A tag must then be located in a very narrowregion (at most, 15° wide in this example) around φ=90° in order toreceive both the first and second part of the data signal.

In principle, there is no lower limit on the width of the data-signalregion obtainable with this method. One may even divide the data signalinto three or more parts and employ three or more scramble beams, aslong as the tags respond only if they receive all parts of the datasignal. This method of reducing the width of the data-signal regionworks also for other the types of antennas described above and in [2].In particular, the method works for planar arrays if the twoscramble-beams nulls are steered in orthogonal directions (planar arraysrequire two scramble beams as explained above and in [2]). Another wayof reducing the width of the data-signal region is to continuously varythe direction of the scramble beam while the data signal is beingtransmitted. Yet another way of reducing the width of the data-signalregion is to increase the power of the scramble beam(s), and therebymove the scramble-beam shoulders above the peak of the data beam.

Systems that employ these highly localized data-signal beams may be usedto locate tags or transceivers with such accuracy that they can replacemore costly laser positioning systems. Such applications are discussedbelow.

A Constant-Level Scramble Beam

A reader can be designed such that its radiated power isomni-directional while its data signal stays highly directional.Consider the 8-element array in FIG. 16 that consists of z-directeddipoles and operates at 2.4 GHz, with element spacing=6.25 cm and thetotal array length=50 cm. The tags to be interrogated are near the (θ,φ)=(90°, 90°) direction.

Typical data and scramble beams for this array are shown in FIG. 17. Allthe zeros of the scramble beam, except the central one at φ=90°, aremoved radially off the Shelkunoff unit circle to points on a circle inthe complex plane of radius 1.06. (The theory related to the Shelkunoffunit circle is described in the book “Antenna Theory and Design” by R.S. Elliot, IEEE Press, 2003.) As a result, the scramble beam has onlyone null (the central one) and stays above the data beam everywhereelse. Notice that the scramble beam “follows” the data beam closely, sothat the power of the scramble beam is extremely low away from a 60°angular region centered on φ=90°. Hence, the reader provides littleenergy to charge or communicate with the tags that are located outsidethis 60° angular region.

FIG. 18 shows a scramble beam that has all its zeros, except the centralone, located on a circle in the complex plane of radius 1.46. Thisscramble beam has an almost constant amplitude away from the centralzero at φ=90°. Hence, it is well suited for charging and communicatingwith any tag located away from φ=90°. The excitation coefficients aregiven above the figure. All the excitation coefficients for the scramblebeam in FIG. 18 are positive except the last one, which equals −1. Thesum of these coefficients equals zero.

The array used in this section operates at 2.4 GHz. The method forcreating a reader with omni-directional power pattern works for anyfrequency that results in electromagnetic wave propagation, and inparticular for the popular RFID frequencies around 900 MHz. Instead ofusing the Schelkunoff unit circle representation to achieve theconstant-level scramble signal, one can use the iterativearray-synthesis methods discussed above and in [2] with appropriate costfunctions. The iterative methods can be used directly to achieveconstant-level scramble beams for ring arrays and planar arrays.

Scanning Array Readers that Employ Triangulation

This section describes the method of the present invention fordetermining the precise location of tags. The method may be explainedwith reference to FIG. 19 where the tags are located inside a room withtwo array readers placed on the walls. The array readers transmit narrowsignal beams surrounded by scramble beams as described above. The beamsare scanned using standard beam steering.

In one embodiment, the data signal simply causes a tag to transmit itstag identification number. In a more advanced embodiment, the datasignal contains the current scan angle and a reader identificationnumber, and the tags respond by re-transmitting that scan angle andreader identification number along with a tag identification number. Iftwo or more tags are present in the region of the data signal at anygiven scan angle, prior art anti-collision methods are employed [1,Chapter 7]. The tag transmissions can be recorded by the readers or byother receivers.

Hence, after the two array readers have completed a scan, a table ispopulated with a field for each tag that contains the scan angles forwhich the tag received a signal beam from each of the two readers. A tagtypically receives the data signal from a reader at more than one scanangle since the scan-angle increments are smaller than the width of thedata-signal beam. For location purposes, one can average the angles toobtain a table for each tag that contains one scan angle for eachreader. This procedure is illustrated in FIG. 19 where the scan anglesfor one of the tags are v₁ and v₂. The location of the tag is easilydetermined from these two angles and the positions of the readers.

To determine the position of tags located directly in the line-of-sightbetween array reader #1 and array reader #2 requires an additional arrayreader. For many applications, however, it is possible to place arrayreader #1 and array reader #2 such that no tag can be located directlybetween the readers, and all tag positions can be determined with arrayreader #1 and array reader #2.

The optimal positions for the readers depend on the spatial distributionof the tags and on possible obstacles that can interfere with thetransmissions. For some indoor applications, it is advantageous to hangfrom the ceiling ring-array readers that each can scan 360 degrees. Theposition of a tag could be determined from the transmission of tworing-array readers, provided the tag is not directly between thosereaders. A combination of ring-array, planar-array, and linear-arrayreaders may be optimal in complicated scenarios.

In the example above, the readers are stationary and the tags areallowed to move around. In certain applications, it is advantageous topermit the readers to move as well. For example, one can record theposition of the inventory of a large warehouse with one or more readersthat move around, provided the location and orientation of the readersare known at all times.

A reader can be any type of active transceiver with a narrow signalbeam, including planar array antennas that have pencil-like signal beamssuitable for 3D location. A tag can be any type of passive or activetransceiver that can be placed on an object whose precise location issought. Such tag-reader systems can replace laser and GPS positioningsystems in certain applications.

For example, if a tag is placed on a moving vehicle and readerscontinuously scan as described above, the tag can continuously transmitthe positions of the vehicle to any receiver within range. Anotherapplication of such precision tag-reader systems is land surveying,where the locations in 3D are sought for surface features in the area ofinterest. Yet another application is remote sensing where the positionof receivers must be known precisely.

The present invention also provides a method for determining theposition of a vehicle in an area where multiple tags are placed at knownlocations on stationary objects such as walls. An RFID reader mounted onthe vehicle can determine the position of the vehicle by recording theangles of at least two tags using triangulation.

Inductive RFID Systems

Inductive RFID systems operate at frequencies below 50 MHz, where thewavelength is much longer than the physical dimensions involved, and thereader and tags are inductively coupled. Precise tag location anduser-defined interrogation regions can be achieved with inductive RFIDsystems as described below.

Consider the tag-reader system in FIG. 20 where the reader is at theorigin and the tags are placed on a line parallel to the x axis given byz=30 cm, y=0. The tag antennas are loops parallel to the x-y plane, sothe tags respond only to z-directed magnetic fields.

The reader employs two small loops that lie in the x-y plane with theircenter points 10 cm apart. The spatial dependence of the magnetic fieldsemitted by such loops can be approximated well by the spatial dependenceof the magnetic fields of z-directed static magnetic dipoles, asdescribed in the reference “Plane-wave theory of time-domain fields” byT. B. Hansen and A. D. Yaghjian, IEEE Press, 1999.

The loops of the reader each transmit two signals, so that the totalsignal transmitted by loop #p is

T _(p)(t)=A _(p) a(t)+B _(p) b(t)

where a(t) is the data signal and b(t) is a scramble signal. A_(p) andB_(p) (p=1, 2, . . . , N) are the excitation coefficients for the dataand scramble signals, respectively. FIG. 20 shows an inductive RFIDreader employing two loops parallel to the x-y plane. The center pointsof the loops are on the x axis at x=−5 cm and x=5 cm. The tags arelocated along the line z=30 cm, y=0 and respond to z-directed magneticfields. For the reader shown in FIG. 20, the coefficients are set asfollows:

-   -   A₁=1 B₁=5 A₂=1 B₂=−5.

FIG. 21 shows the magnitudes of the z-components of the resultingmagnetic fields at the tag locations obtained from the magnetic dipoleapproximation. Also shown in FIG. 21 is the noise floor that determinesthe minimum signal strength required to interrogate a tag. If thescramble signal is turned off, all tags within a 75 cm region respond tothe reader (75 cm is approximately the interrogation width obtained witha reader that employs only a single loop antenna). With the scramblesignal turned on, only tags within a 10 cm region respond to the reader,thereby allowing the precise location of individual tags and reducingtag collisions. The scramble signal can charge and prepare the tags forinterrogation in a scenario where the reader scans along a line.

This example involving a reader that employs two loops demonstrates theuse of data and scramble signals in inductive RFID systems. Optimizationmethods can be employed by those skilled in the art of coil design toobtain loop configurations for which the magnetic field of the datasignal is overshadowed by the magnetic field of the scramble signalexcept in selected regions. See, for example, U.S. Pat. Nos. 5,157,605and 6,557,794 and the references therein. Thus, one obtains inductivereaders with user-specified interrogation zones.

RFID Security

According to [1, Chapter 8], high-security RFID systems should havedefense mechanisms against the following three types of attacks: (1)Unauthorized reading of a data carrier in order to duplicate and/ormodify data. (2) The placing of a foreign data carrier within theinterrogation zone of a reader with the intention of gaining access to abuilding or receiving services without payment. (3) Eavesdropping intoradio communications and replaying the data in order to imitate agenuine data carrier (“replay and fraud”).

As with other types of wireless communication systems, RFID systemscurrently use authentication and encryption methods to defend againstthese attacks. These defense methods have certain inherent weaknesses asdescribed in [2] and [3]. Additional security measures can be obtainedby using the secure transmission and reception techniques described in[2] and [3]. These techniques work for any wireless communicationsystem, including systems that operate in the inductive regime asdemonstrated above.

The scramble signals can prevent an eavesdropper located outside theinterrogation zone from gaining access to the data stream that isbroadcast by the RFID reader. Highly directive antennas can be employedto prevent eavesdropping and unauthorized access to the network. Nullscan be placed in the receiving pattern of the reader to preventunauthorized access to the network.

Method for Optically Displaying Interrogation Zones

This section describes a method for optically displaying theinterrogation zone of an RFID reader. The reader interrogates only tagslocated in the interrogation zone.

An optical source attached to the RFID reader, as shown in FIG. 22,sends out one or more light beams that visualize the interrogation zone.For hand-held readers, the optical source can be a small laser pointerthat transmits a beam in the direction of the center of theinterrogation zone. With this embodiment, the user will see a laser doton an object that is located in the center of the interrogation zone.Hence, the reader can be pointed precisely toward a selected object.With more than one laser pointer, the boundaries of the interrogationzone can be displayed, as illustrated in the following example.

FIG. 23 shows an optical source that sends out two light beams and isattached to a four-element hand-held tag reader that operates atfrequencies around 900 MHz. The element spacing is 10 cm and the totalarray length is 30 cm. This type of tag reader is described in [4]. FIG.23 shows the ratio in dB of the scramble signal and the data signal in a20 meter by 20 meter region of space, color coded with a gray scalecolor map. The data beam dominates in the shaded region, which thereforeis the interrogation zone for the reader.

The optical source transmits two light beams that coincide with theboundaries of the interrogation zone, as shown in FIG. 23 and describedin [5]. In one embodiment, the two light beams are generated by twolaser pointers that produce red dots on objects that are at the edges ofthe interrogation zone. Thus, the user can see which objects are in theinterrogation zone.

Multiple readers that work together can be used with triangulation todetermine the absolute location of tags, as described above and in [4].If optical sources are attached to each reader, the intersection oflight beams shows the absolute position of tags.

A Two Element Reader

FIG. 24 shows a compact RFID reader design that employs a two-elementantenna array, as described in [6]. The antenna elements can be of anytype suitable for broadcasting at the RFID frequencies. The arrayelements can be tilted independently with respect to the array axis.FIG. 25 shows the schematic of RF control electronics for thetwo-element array. Each antenna element is driven by a linearcombination of two RF signals: a data and a scramble signal. The beampatterns for each signal are determined by the weighting coefficientsA₁, A₂, B₁, and B₂. Phase shifts (time delays) can be used to steer thebeam patterns in a specific direction. To achieve a difference patternfor the scramble signal, one can set B₁=−B₂.

FIG. 26 shows the free-space field distributions in the x-y plane whenthe array elements are patch antennas with (1+cos(v)) patterns, where vis the angle between the element normal and the observation point in thex-y plane, displaying the strength of the data beam (Left) and thescramble beam (Right). (See R. J. Mailloux, “Phased Array AntennaHandbook,” Artech House, 1994, Chapter 4.) The elements are located at(x, y, z)=(2 m, 8.3 cm, 0) and (x, y, z)=(2 m, −8.3 cm, 0), with elementnormals pointing in the x direction. The antennas operate at 900 MHzwith weighting coefficients A₁=A₂=B₁=−B₂=1. All time delays are zero sothe beams point in the broadside direction.

The sum pattern carries the data signal and the difference patterncarries the scramble signal. The data signal (FIG. 26 Left) has its peakin the broadside direction where the scramble signal (FIG. 26 Right) hasits null. Assume that the power level of the data signal is adjusted sothat tags in the broadside direction at a distance of 10 m receive justenough power to function, and that the modulated scattering from thesetags can be correctly understood by the reader. The plot on the left inFIG. 27 shows the tag interrogation zone achieved with a reader thatbroadcasts only the data signal. All tags at the edge of theinterrogation zone receive just enough power to function. Tags outsidethe interrogation zone do not receive enough power. The right plot inFIG. 27 shows the interrogation zone obtained when the reader broadcastsboth data and scramble signals. For a tag to function in this mode ofoperation, it must be in a location where two conditions are met: (1)the power of the data signal is sufficient to set off a tag and (2) thedata signal overshadows the scramble signal (the data signal is at least10 dB larger than the scramble signal in the dark shaded area of theright plot in FIG. 27; this 10 dB threshold is an arbitrary figurechosen for illustration purposes only, the actual threshold will dependon the particular system being used). The interrogation zone obtainedwith information steering is much narrower than the interrogation zoneachieved with a standard broadcast scheme. Moreover, the angular extentof the interrogation zone is independent of the power levels of thesignals, provided the ratio of the scramble signal power and data signalpower is kept constant. For example, the interrogation zone in the rightplot of FIG. 27 is achieved with A₁=A₂=B₁=−B₂=1. The same angularinterrogation width can be obtained with A₁=A₂=B₁=−B₂=0.5 (since lesspower is transmitted, however, the interrogation range is reduced).

The angular extent of the interrogation zone can be adjusted by changingthe ratio of the scramble signal power and data signal power. Forexample, A₁=A₂=1 and B₁=−B₂=0.5 would produce an interrogation zone thatis wider than the one in the right plot of FIG. 27. The array excitationcoefficients for the data and scramble signals can thus be adjusted tocreate an interrogation beam that precisely fits an opening of acontainer.

A concrete wall is now placed 4 m from the reader. FIG. 28 shows thetotal field distributions (direct field plus reflected field) for thedata and scramble signals. More specifically, FIG. 28 shows the totalsignal strength of the data beam (Left) and the scramble beam (Right)when the beams are broadcast toward a concrete wall. The fielddistributions have ripples (peaks and nulls) that result from the directand reflected fields being in and out of phase. Close to the reader,however, the direct field dominates. FIG. 29 shows the interrogationzones based on the same value for the required power level that was usedin FIG. 27. The interrogation zone of the data signal alone (FIG. 29Left) has widened because the reflected field from the wall can set offadditional tags. The interrogation zone achieved by broadcasting bothdata and scramble signals (FIG. 29 Right) is much narrower and the powerof the reflected scramble signal prevents additional tags from being setoff. This example illustrates that the two-element reader can work in amultipath environment.

A Three-Element Reader

A standard commercially available reader can be augmented to achieve anarrow well-defined interrogation zone. The standard reader employs oneantenna that broadcasts a single interrogation beam. From the discussionthat follows, it is straightforward to augment standard readers thatemploy multiple antennas.

FIG. 30 shows the antenna array consisting of the antenna of thestandard reader in the middle surrounded by two scramble-signalantennas, where the array comprises three identical patch antennas. Themiddle antenna broadcasts the data signal. The outer antennas broadcastscramble signals. The two outer elements are tilted by the angle α. Thesignals fed to the scramble antennas are 180° out of phase and generatedby a scramble signal generator as shown in FIG. 31. The signal fed tothe middle antenna is simply the signal from the standard reader, whichneed not be modified. The two outer elements are driven by the scramblesignal. As indicated in FIG. 30, the antenna elements can be tilted toachieve the desired interrogation zone as described in [2]. The beampatterns for each signal are determined by the weighting coefficients,as shown in the boxes in FIG. 31.

Consider a special design where the three elements are the 900 MHz patchantennas used above in the two-element reader. The element distance ischosen to be 17 cm, and the scramble signal antennas are tilted by theangle α=30° as indicated in FIG. 30. FIG. 32 shows the resulting fielddistribution of the data beam (Left) and scramble beam (Right). FIG. 33shows the interrogation zones (assuming that the power level is adjustedto achieve a 10 m range) for the standard reader that broadcasts only adata signal (Left) and for the augmented reader in FIG. 31 thatbroadcasts both scramble and data beams (Right). The augmented readerhas a much narrower interrogation zone.

The design in FIG. 31 does not require data and scramble signals to bemixed since each antenna element transmits only a data or a scramblesignal. This design can therefore be a cost-effective embodiment of areader that operates in accordance with the principles of [2].

Using Multiple Sets of Excitation Coefficients to Overcome MultipathEffects

In indoor environments, signals bounce off walls and other objects, sothe field at a given observation point is the sum of signals that havetraveled through different paths. In some areas the multipath fieldcomponents can sum to produce a total field that is too weak tocommunicate with a tag. Further, one must consider areas of low fieldstrength in the scramble signal, which cause the data signal to “leak”out into unintended regions.

FIG. 34 shows a reader that consists of four x-directed dipole antennasthat operate at 900 MHz with the excitation coefficients in FIG. 35.FIG. 35 shows a schematic of RF control electronics for the four-elementarray shown in FIG. 34. Each antenna element is driven by a linearcombination of two RF signals: a data signal and a scramble signal. Thebeam patterns for each signal are determined by the weightingcoefficients (in boxes). Phase shifts (time delays) can be used to steerthe total beam pattern in a specific direction. To interrogate tags thatare placed on items that move on a conveyer belt, the reader broadcaststoward a metal wall 3 m away as shown in FIG. 36. The frame of theconveyer belt is modeled as metal cylinders with a thin-wireapproximation. Floor and ceiling are made of concrete. The fielddistribution is computed with a geometrical optics model that includessingle bounces off the metal wall, floor, ceiling, and cylinders. FIG.37 (Top) shows the interrogation zones for a simple superposition of onedata beam and one scramble beam 50 cm above the conveyer belt where thedata signal is at least 10 dB greater than the scramble. The data signalleaks out in several locations as indicated because multipath effectsproduce areas where the scramble signal is too weak.

This leakage can be eliminated by a modification of the broadcastscheme, which uses complementary scramble signals broadcast from thesame array and creates the narrow interrogation zone shown in the bottomplot of FIG. 37. This new scheme operates as follows: Two complementaryscramble patterns are created, for example, using the set of scramblearray coefficients in FIG. 35 and its mirror image. These two scramblepatterns can be broadcast simultaneously using two different scramblesignals and additional mixing elements in the control electronics.Alternatively, the two signals can be broadcast sequentially during thesame interrogation cycle. The bottom plot of FIG. 37 illustrates hownarrow interrogation zones can be achieved even in severe multipathenvironments with the additional scramble signal.

The data signals can also be affected by multipath making it impossibleto interrogate tags at certain locations. This problem can in some casesbe overcome by broadcasting the data signal with multiple sets ofexcitation coefficients. To avoid interference, the various data beamsshould be broadcast sequentially.

These examples serve to illustrate a general method for reducing theeffect of multipath: broadcast multiple beams with the same purpose(either data or scramble beams) by applying different sets of excitationcoefficients to the array. For non-symmetric excitations coefficients(such as the scramble beam coefficients in FIG. 35), one can employ setsof excitation coefficients that are mirror images of each other. Sets ofexcitation coefficients that steer the beams in slightly differentdirections can also help overcome multipath effects.

Optimal sets of excitation coefficients can be determined from modelingand/or on-site measurements with the following procedure: (1) Set allexcitation coefficients equal to zero except the excitation coefficientfor the first array element, which is set equal to one. (2) Compute ormeasure the one-element field distribution over the desiredinterrogation zones. (3) Repeat this procedure for all other arrayelements to obtain N sets of one-element field distributions for anarray with N elements. (4) Use linear combinations of the one-elementfield distributions to compute the total field distribution when thearray is driven by a particular set of excitation coefficients. (5)Select sets of excitation coefficients so that the combined beams resultin correct interrogation of tags placed at arbitrary locations in theinterrogation zone.

Step (5) of the procedure can be achieved as follows: Start with a firstset of excitation coefficients that would work for free space. Determinethe locations in the interrogation zones where the corresponding fielddistribution is too weak when the reader operates in the multipathenvironment of interest. Determine a second set of excitationcoefficients by modifying the first set of excitation coefficients,which creates a field distribution that fills out the areas where thefield distribution of the first set of excitation coefficients is tooweak. The modification of the first set of excitation coefficients canbe achieved, for example, by slightly changing the phase and by changingthe order of the excitation coefficients.

Creating Well-Defined Interrogation Zones and Preventing ReaderCollisions

This section considers a network of readers and shows how to preventinterference and collisions between readers, as described in [6]. FIG.38 shows two readers that are located in a room with a concrete wall,floor, and ceiling. The readers interrogate tags placed on items thatmove on two conveyer belts. Each reader uses the two-element array ofpatch antennas illustrated in FIGS. 24 and 25. The readers are in closeproximity of each other and the concrete wall. Concrete is modeled by ahomogeneous medium with a relative permittivity of 6 and a conductivityof 0.1 S/m. The field distributions are computed from geometrical opticswith one bounce off each surface included. Polarization, reflectioncoefficients, and geometrical spreading are included in thesecalculations (this simulation and the other simulations are includedsimply for purposes of illustration; a different calculation of thefield can be used in any given configuration to determine the fields towhatever order is needed). The power levels are adjusted so that when areader operates in free space the data signal is just strong enough toset off tags 10 m away from the reader in the main-beam direction, asshown in FIG. 27.

FIG. 39 shows the total field distributions 1.5 m above the concretefloor of the data signals of the left and right reader. The direct andreflected fields being in and out of phase cause the ripples of thetotal field. FIG. 40 shows the interrogation zones of the two readerswhen they broadcast only data signals. A tag placed near the origin willreceive enough power from both readers to be powered up. However, such atag would not function properly, even if the two readers broadcast atdifferent frequencies, because it simultaneously receives two sets ofinstructions (reader collision occurs). Hence, the readers cannotoperate simultaneously. Moreover, both readers interrogate tags on bothconveyer belts, so it would not be possible to determine which beltcarried a tagged item.

For this example, assume that the two readers broadcast both data andscramble signals, and in addition place a scramble signal transmitterbetween the two conveyer belts as indicated in FIG. 38. FIG. 41 (Left)shows the ratio of total scramble beam (obtained with the scrambletransmitter and the two readers) to data beam of the reader on the left.FIG. 41 (Right) shows the ratio of total scramble beam to data beam ofthe reader on the right. Tags are interrogated only in the regions wherea data beam dominates. The data signals dominate in the dark shadedregions. FIG. 42 shows the interrogation zones of the two readers where(1) the power of the data signal is sufficient to set off a tag and (2)the data signal overshadows the scramble signal by at least 10 dB (wherethe value 10 dB is chosen for illustration purposes). The interrogationzones are now disjoint and each reader interrogates only the tags on oneconveyer belt. This aspect of the present invention solves two problems:(1) Reader collision is avoided: tags near the origin stay quiet and donot modify their stored data because they receive a scramble signal thatdoes not instruct them to operate. (2) It is now possible to determinewhich belt carried a tagged item.

More generally, one can set up a reader network with unknown parameters(array excitation coefficients, reader locations, and readerorientations) and optimize the parameters to create desiredinterrogation zones in a given environment. The optimization can becarried out by interactive methods that minimize a user-defined costfunction (see, for example, P. Venkataraman, “Applied Optimization withMATLAB Programming,” Wiley, 2001). This approach is equivalent to aninverse source problem where the task is to determine the strength andlocation of sources that result in a desired field distribution.

One type of solution would determine the optimal source distribution(excitation coefficients) to maximize the signal from a tag placed on aparticular object using the techniques described in the paper by DavidIsaacson entitled “Distinguishability of Conductivities by ElectricCurrent Computed Tomography” (IEEE Trans. on Medical Imaging, Vol. MI-5,No. 2, 91-95, 1986).

In one embodiment of this method, the array element locations are fixedand on-site measurements are carried out to determine the N one-elementfield distributions described above. These N data sets are subsequentlyused in an optimization procedure that determines array coefficientsthat produce the desired interrogation regions.

Optimum Tag Placement

Numerous studies have demonstrated the difficulty of reading tagsaccurately, especially when other objects shield the tags from theinterrogation signal (see, for example, “RFID will present a stifftest,” Supply Chain Management Review, Jan. 15, 2004). This sectiondescribes a systematic method for determining the optimal tag placementthat will maximize the scattered field from the tags. The methodinvolves the following steps:

(1) Create a model of the environment in which the tags must operate.For example, if the tags are to be placed on individual soda bottlesthat are stacked on a pallet, the model would consist of a collection ofstacked high-dielectric scatterers shaped as soda bottles.

(2) Numerically determine the total electric field for the scatteringproblem in which the field of the reader illuminates the model. For thesoda bottles on the pallet, a finite-difference time-domain method wouldbe suitable for determining the total electric field everywhere (A.Taflove and S. Hagness, “Computational Electrodynamics: TheFinite-Difference Time-Domain Method,” Artech House, 2^(nd) Ed., 2000).High-frequency methods (A. K. Bhattacharyya, “High-FrequencyElectromagnetic Techniques,” John Wiley & Sons, 1995) and exactsolutions (W. C. Chew, “Waves and Fields in Inhomogeneous Media,” IEEEPress, 1995) are also useful for solving the scattering problems.

(3) Based on the computed field distribution, place the tag antennassuch that the electric field is disturbed as much as possible. Forexample, if the tag antenna is a linear dipole and the object is a sodabottle, the dipole should be placed at a point on the surface of thebottle where the electric field is strongest. Moreover, the tag antennashould be aligned with the electric field at that point. For sodabottles on a pallet, the optimum tag locations may vary from bottle tobottle.

As shown by the 2D model in FIG. 43, this method works in the followingway: the item to be tagged is modeled by an infinite dielectric cylinderof radius 5 cm with a relative dielectric constant of 81 and aconductivity of 0.01 S/m. The field of a line source (the reader)illuminates a dielectric cylinder (bottle containing a liquid). Aconducting wire (the tag) is close to the surface of the cylinder. Thereader is modeled with an electric line source 5 m away that broadcastsat 900 MHz. Assume that the reader is monostatic: the transmitting andreceiving antennas are collocated. FIG. 44 shows the resulting totalelectric field inside and outside the dielectric cylinder, whosecircumference is indicated by a distinct circle. An optimal tagplacement for this object would be either the front or back (as seenfrom the reader) where the electric field attains its maximum values. Atag placed on the sides of the dielectric cylinder would not scattermuch. The field distributions obtained with a thin wire placed on theside and on the back of the dielectric object are shown in FIGS. 45 and46, respectively. As expected, the wire placed on the back (FIG. 46)alters the field much more than the wire placed on the side (FIG. 45).

To compute the modulated scattered field that would be observed by thereader, one may model the two states of a tag as follows: Ashort-circuited tag antenna is a thin wire, and an open-circuited tagantenna is an absent wire. With this model, a tag placed on the side ofthe dielectric object communicates with the reader by changing the fielddistribution from the one displayed in FIG. 44 (tag antennaopen-circuited) to the distribution in FIG. 45 (tag antennashort-circuited). Similarly, a tag placed on the back of the dielectricobject communicates with the reader by changing the field distributionfrom the one displayed in FIG. 44 (tag antenna open-circuited) to thedistribution in FIG. 46 (tag antenna short-circuited).

The difference fields recorded by the reader far from the dielectriccylinder are shown in FIG. 47 (tag placed on side of cylinder) and FIG.48 (tag placed on back of cylinder). As expected from the discussionabove, the difference field is very weak when the tag is placed on theside of the cylinder, so the tag may not be read correctly. By placingthe tag on the back of the cylinder in accordance with the method of thepresent invention, a much stronger difference field results (at least100 times stronger) and the chances that the reader accurately obtainsthe information stored on the tag greatly improves.

Bistatic RFID Reader Configuration

A bistatic reader configuration is shown in FIG. 49 where thetransmitter and receiver are not collocated. FIG. 49 shows a bistaticRFID reader consisting of a transmitter and a receiver that interrogatesa collection of tags that are placed on items in a box. The differencefield in FIG. 47 attains its maximum values at locations that areapproximately 90° away from the transmitting antenna of the reader.Hence, if the receiving antenna of the reader were placed 90° away fromthe transmitting antenna, the reader would more effectively interrogatethe tag in this configuration where the tag is on the side of thecylinder. Moreover, by separating the transmitter from the receiver, thedirect coupling is significantly reduced and the read range is no longerlimited by the condition that the tag signal may be no more than 100 dBbelow the level of the transmitters carrier signal. (See page 145 ofreference [1] cited above for a discussion of the 100 dB condition.)

This example illustrates two advantages of a bistatic reader over amonostatic reader: (1) a bistatic reader may be able to correctly readcertain tags that cannot be read accurately by a monostatic readerbecause the back scattered field is much weaker than the scattered fieldat an optimal receiver location, and (2) the direct coupling between thetransmitter and receiver is much weaker for a bistatic reader, thusmaking it possible to correctly interrogate tags that are further away.

An even more effective reader would have several receiving antennasdistributed around the objects of interrogation to pick up scatteredfields that peak in many different directions (multistatic reader). Forfixed geometries, such as soda bottles on a pallet, numericalsimulations can determine the optimal bistatic configuration. The use ofnumerical simulations to optimize the placement of tags and readerantennas is illustrated by the example above, which shows that a 90°bistatic configuration is optimal for a tag placed on the side of adielectric cylinder.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made herein without departing from the invention asdefined by the appended claims. Moreover, the scope of the presentapplication is not intended to be limited to the particular embodimentsof the process, machine, manufacture, composition of matter, means,methods, and steps described in the specification. As one will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized. Accordingly, the appended claims areintended to include within their scope such processes, machines,manufacture, compositions of matter, means, methods, or steps.

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 27. A method for improving the efficiencyof RFID systems having RFID readers and RFID tags, comprising the stepsof: employing an antenna array by an RFID reader, broadcasting a datasignal using said antenna array, broadcasting a scramble signal usingsaid antenna array, and adjusting array excitation coefficients of saidantenna array for said data signal and said scramble signal such thatsaid scramble signal overshadows said data signal in all but selectedregions.
 28. The method of claim 27, wherein said antenna array employspatch antennas.
 29. The method of claim 27, further comprising the stepof employing three antenna elements by said RFID reader, wherein twoantenna elements transmit said scramble signal and one antenna elementinterrogates RFID tags.
 30. The method of claim 27, further comprisingthe step of adjusting the array excitation coefficients for said datasignal and said scramble signal to create an interrogation beam thatfits an opening in a container.
 31. The method of claim 27, furthercomprising the step of employing an antenna, wherein said antennatransmits two or more interrogation beams and wherein any of said RFIDtags in said interrogation zone receive sufficient power to operate fromat least one of said two or more interrogation beams.
 32. The method ofclaim 27, further comprising the step of employing two or more scramblesignals, wherein said two or more scramble signals prevent leakage ofsaid data signal.
 33. The method of claim 32, wherein two or morescramble signals employ coefficients that are mirror images.
 34. Themethod of claim 27, further comprising the step of transmitting saiddata signal and said scramble signal adjusted to create closely spaceddisjoint interrogation zones.
 35. The method of claim 27, wherein theposition of said RFID reader is determined from the solution of aninverse source problem.
 36. The method of claim 35, wherein the inversesource problem is solved with an iterative optimization scheme.
 37. Themethod of claim 27, wherein the position of said RFID reader isdetermined from measurements.
 38. The method of claim 27, wherein thearray excitation coefficients are determined from the solution to aninverse source problem.
 39. The method of claim 38, wherein the inversesource problem is solved with an iterative optimization scheme.
 40. Themethod of claim 27, wherein the array excitation coefficients aredetermined from measurements.
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